1. Field of the Invention
This invention relates to a light deflecting electric motor for scanning laser beams used in an optical system of laser printers, digital copiers, etc.
2. Description of the Prior Art
FIG. 6 shows one of conventional light deflecting electric motors of the above-described type. The shown motor comprises a base 1 having a recess 1a. An upper cylindrical protrusion 2 extends upward from the bottom of the recess 1a. A lower cylindrical protrusion 3 concentric with the upper protrusion 2 extends downward from the underside of the base 1. Interiors of the protrusions 2 and 3 serve as a through hole 8. Two bearings 10 are provided in the hole 8 and a rotational shaft 9 is rotatably mounted on the bearings 10 in the hole 8. The shaft 9 has a lower end on which a balancer 11 is fixed by a bolt 11a. A bottom cover 12 is mounted on the bottom of the lower protrusion 3 so as to receive the shaft 9 via the balancer 11 and the bolt 11a. A coil spring 13 is interposed between the lower bearing 10 and the balancer 11 to downwardly urge the shaft 9.
A stator 4 of the motor is fixed on an outer circumferential face of the upper protrusion 2. The stator 4 includes a stator core 5 made of laminated steel sheets, a coil 6 wound on the stator core 5, and a stator yoke 7 fixed on the stator core 5. A mirror mount 15 is mounted on an upper portion of the shaft 9. A polygon mirror 16 is fixed to an upper face of the mirror mount 15. A rotor yoke 17 is fixed to an underside of the mirror mount 15. The rotor yoke 17 includes a rotor magnet 18 disposed opposite the stator 4. The rotor yoke 17 and the rotor magnet 18 constitute a rotor 14 of the motor.
The stator 4 is energized so that the rotor 14 and the polygon mirror 16 are rotated. The polygon mirror 16 is caused to oscillate upon high-speed rotation if one or more of rotating members K including the shaft 9, mirror mount 15, polygon mirror 16, rotor yoke 17 and rotor magnet 18 are unbalanced. The oscillation of the polygon mirror 16 causes a laser beam reflected on it to deviate from a predetermined location, resulting in a deterioration in printing quality in the laser beam printer, for example. Balance adjustment is conventionally carried out for the rotating members K to prevent deterioration in the printing quality. More specifically, the balance adjustment is carried out both on the upper face of the polygon mirror 16 and on the upper face of the balancer 11. This manner is referred to as xe2x80x9ctwo-plane balancing.xe2x80x9d A balancing weight is placed on the upper face of the polygon mirror 16. The upper face of the balancer 11 is cut away or otherwise machined so that the balance is adjusted.
The weight of the polygon mirror accounts for a large percentage of the total weight of the rotating members K. Accordingly, the center of gravity of the rotating members K in the assembled state depends upon a location of the polygon mirror. Since the polygon mirror 16 occupies an upper part of the assembled rotating members K in the above-described motor, the center of gravity of the assembled rotating members K is located between the upper bearing 10 and the polygon mirror 16. When the center of gravity of the rotating members K is thus located above the upper bearing 10, the rotating members K are rotated like a pestle during high-speed rotation while being oscillated. As a result, the oscillation of the rotating members K or more specifically of the polygon mirror 16 is increased.
Further, when the balance adjusting face is located on the upper face of the polygon mirror 16 or on the lower end of the shaft 9, a balance weight is disposed radially or axially away from the center of gravity of the rotating members K or a portion of the member located radially or axially away from the center of gravity of the rotating members K is cut away. Accordingly, even if the rotating members K are balanced as a whole, unbalance in the weight occurs at portions away from the center of gravity of the rotating members K. Thus, when the weight unbalance occurs axially away from the center of gravity of the rotating members K, the polygon mirror 16 is subjected to a large gyroscopic moment thereby to be rotated during the high-speed rotation while being oscillated, whereupon the oscillation is further increased.
Therefore, an object of the present invention is to provide a light deflecting electric motor in which an amount of oscillation of the polygon mirror during the high-speed rotation can be reduced.
The inventions presented herein provide a light deflecting electric motor comprising a stator assembly including a base, a fixed shaft fixed to the base and a stator mounted on the base, a rotor assembly including a rotating member rotatably mounted on a plurality of bearings further mounted on the fixed shaft, a polygon mirror mounted on the rotating member and an outer circumferential surface and a rotor mounted on the rotating member, the rotor assembly having a center of gravity located between the bearings, the rotor assembly being disposed so that a plane which is generally perpendicular to a center of rotation of the rotor assembly and passes the center of gravity of the rotor assembly passes an inside of the polygon mirror, and the balancing plane provided in the vicinity of a plane.
According to the above-described construction, the balancing plane of the rotor assembly is substantially co-planar with a plane passing the center of gravity of the rotor assembly. Consequently, since the occurrence of gyroscopic moment is prevented in the rotor assembly during the high-speed rotation, the rotor assembly can be prevented from being oscillated. Further, the center of gravity of the rotor assembly is located between the bearings. Accordingly, even when unbalanced, the rotor assembly can further be prevented from being oscillated.
In a preferred form of the invention, each bearing comprises a ball bearing including a number of rolling members each made of ceramic. When each bearing comprises the ball bearing, the oscillation of the rotor assembly resulting from the unbalance in the weight thereof can be restrained or limited by the rigidity of each bearing. Further, when each bearing comprises the ball bearing, the rolling members roll with rotation of the rotor assembly such that the rolling members are worn or the temperatures of the rolling members are increased. The wear or temperature increase of the rolling members causes changes in the accuracy of the bearing. However, ceramic is hard to wear and is less adversely affected by heat. Consequently, changes in the accuracy of the bearing can be prevented and accordingly, the service life of each bearing can be improved.
In another preferred form, the rotor assembly has a balancing groove formed in a portion thereof located below the bearings. Since a location where balancing is carried out is axially spaced away from the center of gravity of the rotor assembly, the balancing weight can be reduced.
In further another preferred form, when the rotor is generally annular and includes a rotor magnet radially opposed to the stator with respect to the rotor and a rotor yoke provided on the rotating member to hold the rotor magnet, and the balancing groove is formed in the rotor yoke or the rotating member, or defined between the rotating member and the rotor yoke.
In further another preferred form, the polygon mirror is generally annular and has a reflecting surface, and the balancing groove is disposed inside relative to the reflecting surface of the polygon mirror.